FIELD OF THE INVENTION
This invention relates generally to fiber reinforced resins, and to the use of thermotropic liquid crystalline polymers to improve such resins.
Shaped articles that are made from fiber reinforced thermoplastics typically are formed by a process involving one or more stages wherein the thermoplastic is melted and mechanical forces are applied. As one typical example, the thermoplastic is first melted and then conveyed along an extruder, wherein the fiber is added along with other additives such as processing aids, flame retardants, plasticizers and stabilizers. The fiber reinforced thermoplastic obtained from this first extrusion stage is then subjected to a second melt processing stage such as injection molding, wherein the polymer is melted and forced, under pressure, into a mold. A characteristic of such operations is that the fiber reinforced thermoplastic is subjected to shear forces through the conveying and mixing processes at temperatures near or above the melting point of the thermoplastic. These shear forces result in damage to the fiber, including reduction in fiber length, which reduces the mechanical properties of the article that is eventually obtained. It is generally desirable, therefore, to minimize this fiber damage, in order to provide optimum mechanical properties in the final article produced from the thermoplastic.
Various processing techniques, such as adding the glass fiber as late as possible to the process, are known in the art for reducing damage to the fibers. These techniques are not, however, always compatible with either the processing method being employed, or the equipment used to carry out the process.
This invention concerns the use of thermotropic liquid crystalline polymers ("TLCPs") to minimize fiber damage during processing. The use of TLCPs to reduce the viscosity of thermoplastics is well known in the art. The rheology of thermoplastic/TLCP blends is complex, with numerous examples of significant negative deviations from linear mixing rules. Blends of thermoplastics and TLCP, for example, can have viscosities substantially lower than either component individually. TLCPs have even been used to reduce the viscosity of glass fiber reinforced thermoplastics. Cogswell et al. in U.S. Pat. Nos. 4,433,083 and 4,386,174, for example, reported a 5% viscosity reduction in a polysulfone resin when 10 wt. % of a miscible TLCP was added and sheared at 1000 s.sup.-1.
In the present invention TLCPs are added to fiber reinforced thermoplastics before the thermoplastics are processed in order to minimize the fiber damage that occurs during the processing. In contrast to Cogswell, a TLCP and thermoplastic are chosen that are at least partly immiscible. The addition of TLCP to the thermoplastic before processing minimizes the amount of fiber damage to the fiber reinforced thermoplastic during processing, when compared to the fiber damage that occurs when the fiber reinforced thermoplastic is processed without TLCP. In many instances, the addition of TLCP even increases the viscosity of the fiber reinforced thermoplastic, in further contrast to the teachings of Cogswell.